Rainwater harvesting: An answer to urban flood control
History repeated itself in the evening of June 3, when a thunderstorm hit Accra and brought devastation to the region, with over 150 deaths.
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Up to 250mm of rain fell on the city and the hills surrounding it, with some water finding its way into the Odaw and Onyasia rivers.
Accra has a history of major flooding similar to the June 13, 1997 event, which saw the Odaw and Onyasia overflowing their banks.
Hundreds of small waterfalls also emerged from nowhere and the hillside began to gush with water from springs hidden under rocks and fens, sending a 10 feet high wall of water into town, sweeping residents away from their homes and cars.
Watching the rain approach in Accra makes you catch your breath and it has nothing to do with where you live or the relationship you have with water . The city always waits for the rains impatiently but when they come, it is never ready.
The question people asked was : How much water the Odaw river could take. Rainwater harvesting is therefore a sustainable solution to this complex problem.
Rainwater harvesting
Managing, controlling and making use of rainwater within the vicinity of rainfall is known as rainwater harvesting.
Rainwater harvesting decreases peak flows as well as reduces flooding and the damages resulting from flooding.
As part of urban storm management, rainwater harvesting can be considered a major flood mitigation and management tool from which developing countries such as Ghana could benefit tremendously.
Dr Barnabas Amisigo of the Water Research Institute of the Council of Scientific and Industrial Research (CSIR-WRI) says, rainwater and storm run-off management constitutes one of the several measures that can be exploited to reduce severe erosion common in most communities located on a hilly terrain, or flooding in cities lying on low grounds.
He stated that a wide variety of harvesting systems existed which were associated with various flood control systems, adding that creation of detention basins was one of the mechanisms required for urban run-off or flood protection.
According to him, detention basins were, just like traffic control, points to block or delay the advancement of water from upstream, especially from the mountains till the pressure of the flood downstream eases.
He said because most houses in the city were paved, rain water that should have sunk into the sand found its way into gutters and streets.
He explained that one way to prevent the situation was to store water, stressing the need for gated communities such as estates to incorporate rainwater harvesting system into building plans and create underground storage to be able to collect certain amount of water whenever it rains.
Dr Amisigo suggested that it would be very important to have detention basins incorporated into the country’s drainage system, stating that roads currently under construction should have the appropriate designs to divert the water into a detention basin which could be used for irrigating agriculture lands.
Benefits
It is always forgotten that rainwater caused the first flooding of the earth which destroyed every living creature except those with Noah in the ark he created (Gen 7, 8:1-14).
Flooding, for instance, could be controlled through the creation of reservoirs at lower grounds to collect runoffs.
The runoff water, when collected, can then be transported away as the rain intensifies. This technique could turn the otherwise devastating storm waters into a useful resource in agriculture, thereby contributing to the preservation of the natural environment.
Urbanisation
Currently, the increasing population of the urban areas resulting in the spread of the boundaries of cities present a new dimension to the need to harvest rainwater.
According to Eng. Dr Fredrick Amu-Mensah of CSIR-WRI, natural lands with vegetative cover ensured a balance of run-off flow during rainfall. He said when those lands were cleared for development, their balance was compromised and excessive erosion and run-off flow resulted.
He also said since water flowed from high points to lower points, the low-lying areas of Sakaman and others would increasingly be subjected to flooding because rainwater from as far as Madina and Ashongman eventually ended up at those low lying locations.
“It is therefore, very crucial that rainwater harvesting should be looked at, not only as a means of providing more water for domestic use in every home, but also as a means of reducing flooding.” He said.
Dr Amu-Mensah said if care was not taken, the continuous expansion of urban areas would negatively affect their low-lying areas.
“There is therefore the need for the enforcement of building codes with regard to the provision of rain-water harvesting in new buildings as a prerequisite to obtaining a building permit.” He advised.
The enactment of laws to ensure that estate developers create water storage facilities in their estates to harvest rainwater will be desirable.
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